Device and method for regulating intensity of beam extracted from a particle accelerator

ABSTRACT

The invention concerns a device ( 10 ) for regulating the intensity of a beam extracted from a particle accelerator, such as a cyclotron, used for example for protontherapy, said particles being generated from an ion source. The invention is characterized in that it comprises at least: a comparator ( 90 ) determining a difference ε between a digital signal I R  representing the intensity of the beam measured at the output of the accelerator and a setpoint value I C  of the beam intensity: a Smith predictor ( 80 ) which determines on the basis of the difference ε, a correct value of the intensity of the beam I P ; an inverted correspondence table ( 40 ) supplying, on the basis of the corrected value of the intensity of the beam I P , a setpoint value I A  for supply arc current from the ion source ( 20 ).

SUBJECT OF THE INVENTION

The present invention concerns the technical field of regulating theintensity of a beam extracted from a particle accelerator.

The present invention relates to a device intended for rapidly andaccurately regulating the intensity of a beam extracted from a particleaccelerator, and more specifically a cyclotron.

The present invention also relates to a method for regulating theintensity of the beam extracted from a particle accelerator.

The present invention lastly relates to the use of this device or thismethod in proton therapy, and in particular in the technique of “PencilBeam Scanning”.

TECHNICAL BACKGROUND AND PRIOR ART

Cyclotrons are circular particle accelerators, which are used toaccelerate positive or negative ions up to energies of a few MeV ormore. This type of equipment is employed in various fields such asindustry or medicine, more precisely in radiotherapy for the productionof radioisotopes, or in proton therapy with a view to treating cancertumors.

Cyclotrons generally comprise five main components: the ion source whichgenerates the ionized particles, the device for vacuum confinement ofthe ionized particles, the electromagnet which produces the magneticfield that guides the ionized particles, the high-frequency acceleratorsystem intended to accelerate the ionized particles, and the extractiondevice making it possible to deviate the ionized particles from theiracceleration trajectory then remove them from the cyclotron in the formof a beam with a high kinetic energy. This beam is then directed at thetarget volume.

In the ion source of a cyclotron, the ions are obtained by ionizing agas medium consisting of one or more gases in a closed compartment, bymeans of electrons accelerated strongly by cyclotron electron resonanceunder the effect of a high-frequency magnetic field injected into thecompartment.

Such cyclotrons can be used in proton therapy. Proton therapy isintended to deliver a high dose in a well-defined target volume to betreated, while sparing the healthy tissue surrounding the volume inquestion. Compared with conventional radiotherapy (X-rays), protons havethe advantage of delivering their dose at a precise depth which dependson the energy (Bragg peak). Several techniques for dispensing the dosein the target volume are known.

The technique developed by Pedroni and described in “The 200-MeV protontherapy project at the Paul Scherrer Institute: conceptual design andpractical realization” MEDICAL PHYSICS, January 1995, USA, vol. 22, No.1, pages 37-53, XP000505145 ISSN: 0094-2405, consists in dividing thetarget volume into elementary volumes known as “voxels”. The beam isdirected at a first voxel and, when the prescribed dose is reached, theirradiation is stopped by abruptly deviating the beam by means of afast-kicking magnet. A scanning magnet is then controlled so as todirect the beam at a next voxel, and the beam is reintroduced so as toirradiate this next voxel. This process is repeated until all of thetarget volume has been irradiated. One of the drawbacks of this methodis that the treatment time is long because of the successive stops andrestarts of the beam between two voxels, and may be as much as severalminutes, in typical applications.

Patent application WO00/40064 by the Applicant describes an improvedtechnique, referred to as “pencil beam scanning”, in which the beam doesnot have to be stopped between the irradiation of each individual voxel.The method described in this document consists in moving the beamcontinuously so as to “paint” the target volume layer by layer.

By simultaneously moving the beam and varying the intensity of thisbeam, the dose to be delivered to the target volume can be configuredprecisely. The intensity of the proton beam is regulated indirectly byaltering the supply current of the ion source. To this end, a regulatoris employed which makes it possible to regulate the intensity of theproton beam. This regulation, however, is not optimal.

Another technique used in proton therapy is the technique referred to as“Double Scattering”. In this technique, the irradiation depth (i.e. theenergy) is modulated with the aid of a wheel, referred to as amodulation wheel, rotating at a speed of the order 600 rpm. Theabsorbent parts of this modulator consist of an absorbent material, suchas graphite or lexan. When these modulation wheels are manufactured, thedepth modulation which is obtained is fairly close to predictions. Theuniformity nevertheless remains outside the desired specifications. Inorder to achieve the specifications in respect of uniformity, ratherthan re-machining the modulation wheels it is less expensive to employbeam intensity regulation which is synchronized with the speed ofrotation of the energy modulator. The modulation function is thereforeestablished for each energy modulator, and is used as a trajectory whichis provided as a setpoint to the beam intensity regulator. Rapid andaccurate regulation of the intensity of the beam extracted from aparticle accelerator is therefore also necessary in the doublescattering techniques which use such a modulation wheel.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a device and amethod intended for regulating the intensity of a beam extracted from aparticle accelerator, which does not have the drawbacks of the methodsand devices of the prior art.

SUMMARY OF THE INVENTION

The present invention relates to a device for regulating the intensityof the beam extracted from a particle accelerator, such as a cyclotron,used for example for proton therapy, said particles being generated froman ion source, characterized in that it includes at least:

a comparator, which determines a difference between a digital signalrepresentative of the beam intensity measured at the output of theaccelerator and a setpoint value of the beam intensity;

a Smith predictor, which determines a corrected value of the beamintensity on the basis of said difference;

an inverted correspondence table, which provides a setpoint value forthe supply of the arc current of the ion source on the basis of thecorrected value of the beam intensity.

The device according to the invention may furthermore comprise ananalog-digital converter, which converts the analog signal directlyrepresentative of the beam intensity measured at the output of theaccelerator and provides a digital signal.

The device according to the invention will preferably furthermorecomprise:

a lowpass filter, which filters said analog signal directlyrepresentative of the beam intensity measured at the output of theaccelerator and provides a filtered analog signal;

a phase lead controller, which samples said filtered analog signal,compensates for the phase lag introduced by the lowpass filter andprovides a digital signal to the comparator.

The device of the invention advantageously includes means for updatingthe content of the inverted correspondence table.

The sampling frequency is preferably between 100 kHz and 200 kHz, andthe cutoff frequency of the lowpass filter is preferably between 2 and 6kHz.

The present invention also relates to a method for regulating theintensity of the beam extracted from a particle accelerator, such as acyclotron, used for example for proton therapy, said particles beinggenerated from an ion source, by means of a digital regulation deviceoperating at a given sampling frequency, characterized in that itcomprises at least the following stages:

the beam intensity is measured at the output of the particleaccelerator;

a digital signal representative of the measurement of the beam intensityis compared with the setpoint value of the beam intensity;

a corrected value of the beam intensity is determined by means of aSmith predictor;

a setpoint value for the supply of the arc current of the ion source isdetermined, on the basis of said corrected value of the beam intensity,by means of an inverted correspondence table.

In the method according to the invention, after the measurement of thebeam intensity at the output of the particle accelerator, the analogsignal directly representative of the measured beam intensity ispreferably converted by means of an analog-digital converter in order toobtain a digital signal.

According to one embodiment of the method according to the invention,

the analog signal directly representative of the measured beam intensityis filtered by means of a lowpass filter, giving a filtered analogsignal;

the filtered analog signal is sampled, and the phase lag introduced bythe filtering is compensated with the aid of a phase lead controller, inorder to obtain a digital signal.

The correspondence between a value for the supply of the arc current ofthe ion source and a value of the beam intensity measured at the outputof the accelerator is advantageously determined prior to the regulation.

In the correspondence between a value of the beam intensity measured atthe output of the accelerator and a value for the supply of the arccurrent of the ion source, the values of the supply of the arc currentcorresponding to the beam intensity values higher than a limit areadvantageously replaced by the supply value of the arc currentcorresponding to this limit.

The present invention lastly relates to the use of the device and themethod of the invention in proton therapy, and in particular in thetechniques of “Pencil Beam Scanning” and “double scattering”.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents a device for regulating the intensity of a beamextracted from a particle accelerator according to the prior art.

FIG. 2 represents the characteristic of the system, i.e. thecorrespondence between a value I_(A) for the supply of the arc currentof the ion source and a value I_(M) of the beam intensity measured atthe output of the accelerator.

FIG. 3 represents one embodiment of a device for regulating theintensity of a beam extracted from a particle accelerator according tothe invention.

FIG. 4 represents a second embodiment of a device for regulating theintensity of a beam extracted from a particle accelerator according tothe invention.

PROBLEMS ON WHICH THE PRESENT INVENTION IS BASED

The problems described below are encountered when using conventionalregulation, for example PID, to carry out the technique referred to as“pencil beam scanning”, as described in the publication WO00/40064 bythe Applicant.

As shown by FIG. 1, a setpoint value I_(C) of the beam intensity isprovided to a conventional PID regulator 10, which determines a valueI_(A) of the arc current of the ion source 20. The beam intensity ismeasured by means of an ionization chamber 30, and the correspondingsignal I_(M) is compared with the setpoint value I_(C) with the aid of acomparator 90, in order to provide an error signal ε. According to thetechnique of continuous beam scanning, it is essential for the beamintensity to vary simultaneously with the movement, so as to obtainconformity of the delivered dose.

Such a system has the following difficulties:

a significant pure dead time is due to the long transit time of aparticle between its emission by the ion source 20 and its exit from themachine;

the characteristic of the system; which relates the intensity of thebeam extracted from the particle accelerator I_(M) to the strength ofthe arc current of the ion source I_(A), is very nonlinear as shown byFIG. 2;

this characteristic may furthermore vary with time, as shown by thedashed curves in FIG. 2. This variation may take place rapidly becauseof the heating or cooling of the filament of the ion source when it isput into operation. It may also be due to the ageing of the filament.These two phenomena lead to variations of the characteristic with verydifferent time constants;

the system is very noisy. The intensity of the beam generated by the ionsource has significant noise, in particular at the sampling frequencywhich is used for the measurement.

The regulation of such a system by using the conventional regulationmethods, such as the techniques of feedforward, feedback byproportional, integral and derivative action (PID) and cascade loops,was evaluated. Because of the significant pure dead time, all thesemethods give responses which either are too slow or are unstable. Nor dothe conventional methods make it possible to address the problem of asystem characteristic that fluctuates as a function of time, by using anaverage value of the characteristic over a given period, because thegain variations from one response to the other are in a very largeratio.

The variation of the characteristic depends on two phenomena which arevery much decoupled: the first, with a short time constant, correspondsto the conditioning of the ion source, i.e. its temperature. Normaloperation, continuous or intermittent with a high duty cycle, heats theion source rapidly. This fast temperature establishment time mightpermit open-loop operation, i.e. without taking the actualcharacteristic of the system into account, by using conventional methodsduring the conditioning time. However, this compromise greatly limitsthe use of a conventional method with intermittent operation at a mediumduty cycle, which often corresponds to the operating mode that is used.

The second phenomenon, with a longer time constant, is due to the ageingof the filament and the ion source itself. This slower change in thecharacteristic could therefore occasion the use of an averagecharacteristic of the system. However, the use of an averagecharacteristic leads to a regulation which either is too slow or isunstable.

It therefore seems clear that the conventional regulation methods cannotsatisfactorily resolve the problems of regulating such a system, i.e. apure dead time which is much longer than the main time constant of thesystem (about 4 times) and a variable nonlinear characteristic thatrequires an adaptive regulation method.

Rapid and accurate regulation of the intensity of the beam extractedfrom a particle accelerator is therefore confronted with manydifficulties. However, such rapid and accurate regulation is importantfor using the “pencil beam scanning” technique.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The present invention consequently proposes to resolve this problem morespecifically by using, according to a preferred embodiment, theregulation device 10 represented in FIG. 3 with the supply of the arccurrent of the ion source 20. The ion source produces an ion beam, whichis accelerated during its transit through the accelerator, is extractedtherefrom and passes through a device 30 for measuring the beamintensity at the output of the accelerator. This measuring device 30may, for example, be an ionization chamber.

The regulator according to the invention was used for a cyclotron havingthe following exemplary and nonlimiting characteristics:

-   -   fixed energy: 235 MeV    -   pure dead time: 60 μsec. This pure dead time corresponds to the        transit time of the ions through the accelerator. It therefore        corresponds directly to the time required for measuring the        effect of a modification of the setpoint of the arc current of        the ion source on the intensity of the ion beam extracted from        the machine.    -   main time constant: 15 μs. This gives an indication of the time        required for establishing the response of the system to a        setpoint modification in an open loop.    -   very nonlinear characteristic of the system, which leads to an        open-loop characteristic corresponding substantially to that of        a system with a hybrid dynamic response (all or nothing).    -   variation of the characteristic with time.    -   very noisy measured signal. This is because the ion source is        unstable, which leads to a very high noise level for the        intensity of the beam after extraction. The observed        noise/signal ratio is of the order of 150%. For a digital        embodiment of the regulator, the adopted sampling frequencies        therefore lead to a very low signal/noise ratio.

In the regulation device of the invention, which is represented in FIG.3, the following stages are carried out:

-   -   the setpoint value of the beam intensity I_(C) is provided in        the form of a 0-10 V analog signal (10 V corresponding to a beam        intensity of 300 nA);    -   the beam intensity is measured by means of an ionization chamber        30, and the measurement I_(M) is provided to the regulation        device 10 by means of a 0-15 μA analog signal (15 μA        corresponding to a beam intensity of 300 nA);    -   this analog signal I_(M) is converted into a digital signal        I_(R) by a converter 50;    -   this signal I_(R) is compared with the setpoint I_(C) by the        comparator in order to provide an error signal ε;    -   this error signal ε is provided to the regulator 80 of the        “Smith predictor” type;    -   the output I_(P) of the Smith predictor 80 is then provided to        the input of an inverted correspondence table 40. The        correspondence table 40 numerically provides the nonlinear        relation between the arc current of the ion source I_(A) and the        intensity of the ion beam I_(M) extracted from the accelerator.        It therefore makes it possible to identify the nonlinear        characteristic of the system. The output of the inverted        correspondence table is converted into an analog signal of the        4-20 mA type I_(A), which is provided by the regulation device        10 as a value of the setpoint for the supply of the arc current        of the ion source.

Simulations show that such a device allows good regulation. It is,however, sensitive to low-frequency perturbations. In order to resolvethis problem, a preferred variant of the device according to theinvention has been developed, which is represented in FIG. 4. In thisdevice 10, a lowpass filter 60 and a phase lead controller 70 areintroduced into the feedback. The filter 60 is, for example, afirst-order lowpass filter. The cutoff frequency is 4.5 kHz. In order tocompensate for the phase lag introduced by the filter, a phase leadcontroller 70 is used (filtered derivator) which compensates for thisphase shift.

Both the device in FIG. 3 and the one in FIG. 4 have an invertedcorrespondence table 40. The content of this table 40 is determinedprior to each use of the device, in the following way:

-   -   since the regulator is in an open loop, the setpoint of the arc        current of the ion source 20 is increased progressively from 0        to 20 mA in the form of a 100 ms ramp;    -   the beam intensity is measured for each of the 4000 sampled        points;    -   the table which is obtained is inverted, so as to provide a        corresponding value of the arc current of the ion source I_(A)        as a function of the beam intensity I_(M).    -   This inverted table is loaded into the regulation device 10.

In practice, this operation is carried out twelve or so times insuccession. This makes it possible to ensure that the parameters reach aplateau corresponding to the steady-state temperature of the filament.In order to eliminate the noise, an average of the last 4 tables iscalculated. These operations, which are carried out automatically, lastat most 1.5 s. In a variant of the invention, the values of I_(A)corresponding to the values of I_(M) higher than a given limit arereplaced by the value of I_(A) corresponding to this limit. The curvesin FIG. 2 are therefore clipped. This is a safety element making itpossible to guarantee that the intensity of the beam produced by theaccelerator will never be more than this limit.

The device according to the invention is produced by means of anelectronics board which employs digital technology of the DSP type(Digital Signal Processing).

The synthesis of the Smith predictor was carried out in the Laplacedomain, and the discretization is provided by the Z transform using themethod of pole-zero correspondence. over-sampling might have beenadequate to avoid any problem associated with the discretization, butcurrent DSP technology did not allow us to go beyond 100 kHz.

The regulation method according to the present invention has severaladvantages. First, it allows controlled adaptation, i.e. it requires avery short computation time compared with modern adaptive controlmethods and allows a very straightforward structural change since theidentification is carried out by constructing a correspondence table,which is then sufficient to invert numerically in order to linearize thecharacteristic of the system seen by the main regulator.

It furthermore offers significant flexibility since it could be employedfor accurate, reproducible, robust and high-performance regulation ofany ion source with which a cyclotron is equipped, and especiallythrough the advantage of adaptive-type regulation allowingre-identification of the characteristic of the system when this varieswith time. It therefore allows the identification and regulation of anaccelerator other than the C235 cyclotron for which this regulation wasoriginally developed.

1. A device (10) for regulating the intensity of the beam extracted froma particle accelerator, such as a cyclotron, used for example for protontherapy, said particles being generated from an ion source,characterized in that it includes at least: a comparator (90), whichdetermines a difference ε between a digital signal I_(R) representativeof the beam intensity measured at the output of the accelerator and asetpoint value of the beam intensity I_(C); a Smith predictor (80),which determines a corrected value of the beam intensity I_(P) on thebasis of the difference ε; an inverted correspondence table (40), whichprovides a setpoint value I_(A) for the supply of the arc current of theion source (20) on the basis of the corrected value of the beamintensity I_(P).
 2. The device as claimed in claim 1, characterized inthat it furthermore comprises an analog-digital converter (50), whichconverts the analog signal I_(M) directly representative of the beamintensity measured at the output of the accelerator and provides adigital signal I_(R).
 3. The device as claimed in claim 1, characterizedin that it furthermore comprises: a lowpass filter (60), which filtersthe analog signal I_(M) directly representative of the beam intensitymeasured at the output of the accelerator and provides a filtered analogsignal I_(F); a phase lead controller (70), which samples the filteredanalog signal I^(F), compensates for the phase lag introduced by thelowpass filter (60) and provides a digital signal I_(R) to thecomparator (90).
 4. The device as claimed in claim 1, characterized inthat it includes means for updating the content of the invertedcorrespondence table (40).
 5. The device as claimed in claim 1,characterized in that the sampling frequency is between 100 kHz and 200kHz.
 6. The device as claimed in claim 1, characterized in that thecutoff frequency of the lowpass filter (60) is between 2 and 6 kHz. 7.Use of the device as claimed in claim 1 in proton therapy, and inparticular in the techniques of “Pencil Beam Scanning” and “doublescattering”.
 8. A method for regulating the intensity of the beamextracted from a particle accelerator, such as a cyclotron, used forexample for proton therapy, said particles being generated from an ionsource (20), by means of a digital regulation device (10) operating at agiven sampling frequency, characterized in that it comprises at leastthe following stages: the beam intensity (I_(M)) is measured at theoutput of the particle accelerator; a digital signal I_(R)representative of the measurement of the beam intensity (I_(M)) iscompared with the setpoint value I_(C) of the beam intensity, by meansof a comparator (90); a corrected value of the beam intensity I_(P) isdetermined by means of a Smith predictor (80); a setpoint value I_(A)for the supply of the arc current of the ion source (20) is determined,on the basis of the corrected value I_(P) of the beam intensity, bymeans of an inverted correspondence table (40).
 9. The regulation methodas claimed in claim 8, characterized in that, after the measurement ofthe beam intensity at the output of the particle accelerator, the analogsignal I_(M) directly representative of the measured beam intensity isconverted by means of an analog-digital converter (50) in order toobtain a digital signal I_(R).
 10. The method as claimed in claim 8,characterized in that after the measurement of the beam intensity at theoutput of the particle accelerator: the analog signal I_(M) directlyrepresentative of the measured beam intensity is filtered by means of alowpass filter (60), giving a filtered analog signal I_(F); the filteredanalog signal I_(F) is sampled, and the phase lag introduced by thefiltering is compensated with the aid of a phase lead controller (70),in order to obtain a digital signal I_(R).
 11. The method as claimed inclaim 8, characterized in that the correspondence between a value I_(A)for the supply of the arc current of the ion source (20) and a valueI_(M) of the beam intensity measured at the output of the accelerator isdetermined prior to the regulation.
 12. The method as claimed in claim8, characterized in that, in the correspondence between a value I_(M) ofthe beam intensity measured at the output of the accelerator and a valueI_(A) for the supply of the arc current of the ion source, the values ofI_(A) corresponding to the values of I_(M) higher than a limit arereplaced by the value of I_(A) corresponding to this limit.
 13. Use ofthe method of as claimed in claim 7 in proton therapy, and in particularin the techniques of “Pencil Beam Scanning” and “double scattering”.